This invention relates to phase locked loop frequency synthesizers and particularly to such frequency synthesizers having a relatively high error sampling rate.
Phase locked loop frequency synthesizers are generally used in applications requiring a highly stable frequency source whose output frequency can be easily changed to one of several integral multiples of a given reference frequency. Briefly, a phase locked loop is comprised of a voltage controlled oscillator which generates the selected output frequency in response to an error signal applied thereto. The error signal is obtained by dividing the output frequency by a selected integer in a frequency scaler and comparing the resultant to the reference frequency in a phase comparator to obtain the error signal. In essence the output frequency f.sub.o is related to the reference frequency f.sub.r as folows: EQU f.sub.o =Nf.sub.r
where N is the selected integer. Thus, the phase locked output frequency is changed by simply changing N, the divider ratio of the frequency scaler.
The above mentioned means for generating multiples of the given reference frequency works well for many applications. Serious limitations, however, emerge when short frequency acquisition time and high spectral purity are required. To begin with, the voltage controlled oscillator is generally a non-linear device. This, coupled with the means typically used to implement a phase locked loop, creates a situation where it is difficult to lock the voltage controlled oscillator rapidly to different multiples of the reference frequency while maintaining a stable loop. Thus, it usually takes a number of reference frequency periods to acquire loop lock. The reason for this can be explained by considering the means for comparing phase difference between the scaled output frequency and the reference signal. In general, phase measurement is made according to one of several techniques, all of which are based upon a somewhat restrictive phase sampling scheme. More specifically, phase error measurements are made at discrete points in time and are delivered as error signals at a rate which is roughly equal to the reference frequency. What results, to a close approximation, is a sample data feedback system which operates at the reference frequency.